Anaerobic membrane bioreactors (AnMBRs) show great promise for wastewater treatment. However, membrane fouling is still a major limiting factor for industrial application. There is little information focusing on membrane fouling of AnMBRs treating low-strength municipal wastewater. The food-to-microorganism (F/M) ratio is a controllable parameter found to significantly influence system performance and fouling in aerobic membrane bioreactors (MBRs). In this study, two lab-scale AnMBRs (named as HAnMBR and LAnMBR) were comparably run under high and low F/M ratios of 3.8 g COD/g MLSS.d and 0.1 g COD/g MLSS.d, respectively. The impact of the F/M ratio on the performance of AnMBRs was systematically evaluated, especially with regard to fouling. The results showed that cake resistance was responsible for over 98% of the total fouling. and membrane fouling in the HAnMBR was more severe than that in the LAnMBR. HAnMBR had higher amounts of soluble microbial products and higher tightly-bound to loosely-bound extracellular polymeric substance ratio in its cake layer accounted for a higher cake resistance. The larger amount of fine particles in the HAnMBR also contributed to more serious fouling. Membrane filtration deteriorated sludge bio-flocculation, which in turn accelerated fouling. (C) 2012 Elsevier B.V. All rights reserved.